The invention relates generally to electron spectroscopy. In particular, the invention relates to a method of determining the intensity of an electron spectroscopic peak.
Many technologically advanced devices rely upon composite structures having a very thin, substantially planar film covering a substrate of another material. An example of such a device is a magnetic recording or read head which has an active surface layer of a ferromagnetic material. High-performance ferromagnetic materials based on, for example, heavier elements such as cobalt, are often brittle and subject to oxidation so that it is common practice to cover the ferromagnetic layer with a very thin protective layer, often of a carbon-based material such as diamond. However, the performance and durability of such devices depend on the manufacturing process to produce a uniform covering layer of diamond with a limited fraction of impurities. Excessive impurities would degrade the ability of the covering layer to protect the underlying ferromagnetic film.
Auger electron spectroscopy, to be described below, is commonly used for determining the composition of surface layers, often of thickness of 6 nm and less. Auger spectroscopy is a type of electron spectroscopy relying upon complex atomic interactions. Briggs et al. have edited a complete reference of Auger and other electron spectroscopy in Practical Surface Analysis, vol. 1, Auger and X-ray Photoelectron Spectroscopy, 2nd ed., (Wiley, 1990). In the typical practice of Auger spectroscopy, the solid is probed with an electron beam in the low keV range of energies and produces a secondary electron through an Auger transition process having a well defined Auger energy EAUGER. In Auger spectroscopy, the probing radiation ejects an inner-shell electron from an atom. Then in the Auger transition, a first outer-shell electron falls into the inner-shell vacancy and a second outer-shell electron is ejected carrying the difference in energy. The spectrometer analyzes the energy of the ejected electron as the Auger energy EAUGER. The Auger energy EAUGER is for the most part unique for each atom, primarily dependent upon the atomic number Z. Thus, the measured electron energy can be used to determine the composition of the material, at least near its surface. These energies are generally in the range of a few hundred eV to a few keV for the typical practice of Auger electron spectroscopy. Usually to enhance the Auger signal, the primary energy Ep is made twice or more the Auger energy EAUGER. Auger electron spectroscopy allows the very quick and highly accurate measurement of film thicknesses up to about 30 nm. Other types of electron spectroscopy are possible with similar equipment, and the technology is close to electron microscopy.
A generic electron spectrometer is schematically illustrated in FIG. 1. Other geometrical relationships may be used. An electron gun 10 emits a primary radiation beam 12 of electrons of energy Ep towards a sample 14 under test, which is supported on a holder 16. An electron energy analyzer 18 receives a beam 20 of secondary electrons emitted from the sample 14 and characterized by energy Es. The low electron energies require that the entire analyzer be operated at very high vacuum levels. The secondary beam 20 tends to be spatially very broad. The electron energy analyzer 18 typically has a spatially fixed entrance slit 22 to fix the angle between the analyzer 18 and the sample 14, and it internally analyzes the secondary energy Es by means of a electrostatic retarder or a magnetic analyzer or other means. Although in some automated applications, the electron analyzer 18 outputs a small number of experimentally determined parameters, the typical analyzer at some level outputs an energy spectrum from which the energy location of one or more peaks is extracted. Modern spectrometers are typically operated under software control by a computer 24, which stores spectrometer data as it is being generated in a memory 26. This design allows the computer 24 to intensively analyze the the entire spectrum after it has been accumulated 26. Such electron spectrometers are well known, very often as Auger or ESCA spectrometers, and are commercially available from several sources, including Physical Electronics (PHI), a division of Perkin-Elmer of Eden Prairie, Minn., Vacuum Generators of the United Kingdom, and Omicron of Delaware.
A major experimental effect in electron spectroscopy is background noise introduced by elastic and inelastic scattering of the primary electrons as they enter the material being tested and scattering of secondary electrons as they pass through the material between their points of interaction with the constituent atoms of the material and the surface of the material. All electrons experience both elastic and inelastic collisions. Inelastically scattered electrons have a wide distribution of energies beginning at the energy Ep of the probing beam and extending downwardly. The elastically scattered spectrum is typically larger because of the small Auger cross sections.
Primary electrons used for Auger spectroscopy typically have energies of a few keV while the Auger transitions are typically below 1 keV. A 1 keV electron has a mean free path in a solid of about 3 nm; a 3 keV electron, 15 nm. Furthermore, secondary Auger electrons are subject to the same type of inelastic scattering. Many technical articles have attempted to explain and quantify the effects of inelastic scattering in order to extract the Auger spectrum. Elastic scattering depends upon the average atomic number Z of the material and is stronger in materials with higher Z.
Auger spectroscopy may be used for two different purposes in determining the purity of a layer. The energy of the Auger peaks can be easily identified with the atomic number of a constituent of the film. Thereby, the atomic composition of the impurity can be relatively easily identified, that is, whether it is nitrogen or iron, for example in a carbon film. The more demanding task is to use Auger spectroscopy for determining a concentration of the impurity, a capability enabled by the fact that the size of the Auger peak increases with the concentration of the particular impurity. However, Auger spectroscopy is poorly suited for compositional measurements in otherwise poorly characterized samples, particularly those of complex composition, because the Auger peak is almost always only a small fraction of the background signal mostly originating from the elastic scattering of the primary electrons.
The conventional method for extracting an Auger peak and its magnitude takes advantage of the fact that the background signal tends to change slowly with the electron energy, as schematically graphed in FIG. 2, while the Auger peaks 30 represent narrow features located on an energy that is unique for each chemical element. Therefore, one of various techniques is used to determine the derivative of the electron intensity spectrum N(E) with respect to energy, that is dN(E)/dE, as schematically graphed in FIG. 3 for the same data. The slowly changing background nearly disappears in the derivative data facilitating the measurement of the signal intensity. In the past, analog methods were used for synchro-detection of the intensity with respect to a dither signal applied to the energy. On the other hand, in modem spectrometers, a complete energy spectrum N(E) is measured and stored in computer memory, and numerical methods are used to produce the differentiated spectrum. Conventionally, the intensity of the differentiated spectrum is measured as the difference amplitude 38 of the signal between its maximum value 34 and minimum value 36. However, this method lacks precision as it depends on the shape of the peak, which in turn depends on the spectral resolution of the spectrometer and chemical state of the emitting atom. Besides, even in the absence of the Auger signal, the amplitude of the derivative is not zero due to the statistical variation of the signal and electronic noise. Several effects contribute to the result that the derivative of the peak 30 assumes a non-zero value 32. Furthermore, Auger transitions in many chemical elements form a series of overlapping peaks, and the separation of overlapping derivative peaks is cumbersome.
Another method for measuring the intensity of an Auger peak integrates the area under the Auger peak. This method requires that the Auger peaks be separated from the inelastic background. Because each electron inside the solid produces an avalanche of inelastically scattered electrons, the low energy side of Auger peaks gradually merges with the inelastic background. Furthermore, to increase the sensitivity of this method, it is frequently desirable to decrease the resolution of the spectrometer. In industrial applications, such low resolution spectrometers enjoy reduced cost. Under such circumstances, the separation of the Auger peak from the background is rather difficult, especially in the case of low intensity Auger peaks and Auger peaks arising from low-level impurities.
Accordingly, it is desired to provide a more accurate method in Auger spectroscopy and other electron spectroscopy of measuring small compositions of impurities. It is particularly desired to measure small compositions of impurities in carbon films.
A method and apparatus for extracting peak intensities in an electron analyzer, especially useful for determining compositions by means of Auger spectroscopy. An electron spectrum is measured on either side of the anticipated peak position, for example, an Auger peak of one of the known constituents of the sample being measured. The data on either side of the peak are fit to respective function relationships, typically linear equations depending upon the electron energy. However, the data close to the peak and experiencing significant variations with energy are not fit. The difference in the two functional relationships near the anticipated peak is associated with the peak intensity and hence the composition of the element producing the peak.
The invention is particularly useful in determining concentrations of a known impurity in thin films of simple composition, for example, nitrogen in carbon film.